Heredity: Chromosomes, DNA and Genes

• Inheritance: Transmission of genetic information from one generation to the next, leading to continuity and variation.
• Chromosomes: Long threads in the nucleus of cells, which contain genetic information.

Chromosomes

• Chromosomes normally appear very thin and are best viewed by electron microscope
• During cell division they shorten and thicken so they can be observed with a standard microscope.
• Typical human cell contains 46 chromosomes, arranged in 23 pairs.
• Sex cells (sperm and ova) have only 23 chromosomes.
• Sex chromosomes make up one out of the 23 pairs in human cells, and determine the the sex of the individual.
• Males have XY chromosomes, while females have XX chromosomes, the Y chromosome leads to the development of male traits.

DNA and Genes

• Each chromosome contains a molecule of DNA.
• The DNA instructs a cell to produce specific proteins
• Instructions for making different proteins are found on each chromosome.
• Genes are the segments that comprise each chromosome
• Genes code for proteins and give us characteristics.
• Genes occupy a uniform relative position on the chromosome.

Alleles

• Alleles are different forms of a gene.
• Alleles appear on paired chromosomes in the same location.
• Monohybrid cross: Crossing of individuals and the study of one character (e.g., flower color) with different traits (e.g., red or white) in their offspring.
• Punnett squares can be used to predict genotypes and phenotypes of offspring in Mendelian genetics.
• Mendel crossed true-breeding pea plants, including those with purple and those with white flowers.

Key Terminology

• Chromosome: Thread of DNA, made up of genes.
• Allele: An alternative form of a gene occupying the same relative position on chromosome pairs.
• Gene: A section of DNA that codes for a protein and controls a specific characteristic.
• Haploid nucleus: Nucleus with a single set of unpaired chromosomes (e.g., sperm and ova), 23 in humans.
• Diploid nucleus: Nucleus with pairs of chromosomes, in somatic (body) cells, 46 in humans.
• Genotype: Genetic make-up of an organism (e.g., Tt, where T and t are alleles of a gene).
• Phenotype: Visible characteristics of an organism, that is controlled by the genotype.
• Homozygous: A pair of identical alleles controlling the same characteristics (e.g., TT).
• Heterozygous: A pair of dissimilar alleles for a characteristic (e.g., Tt).
• Dominant: A gene (e.g., T) that always shows in the phenotype of an organism, whether heterozygous or homozygous.
• Recessive: A gene (e.g., t) that only has an effect on the phenotype when the organism is homozygous.

Phenotypes and Genotypes of Offspring

• In the F1 generation, all seeds were purple, the white flower trait did not appear.
• When true-breeding plants are crossed, purple flowers mask the white trait, so the purple flower is dominant and white flower is recessive.

Crosses

• Cross 1: Crossing a tall and dwarf pure-breeding pea plant.
  • The genotype of the tall pure-breeding plant is TT.
  • The genotype of the dwarf pure-breeding plant is tt.
• Cross 2: Crossing two heterozygous tall pea plants.
  • Both plants have the Tt genotype.
• Cross 3: Crossing two heterozygous tall allele pea plants.
  • The tall alleles are Tt.
  • The dwarf alleles are tt.

Fertilization

• Gametes are sex cells eg sperm and egg.
• Fertilization is the joining together of an egg and sperm to form a zygote.
• Subsequent cell divisions in the zygote creates an embryo.
• Mother's gametes consist of: X and X
• Father's gametes: X and Y.
• 50% probability of producing a female, and 50% probability of producing a male.
• Fertilization is random, as numerous sperm cells have the potential to fertilize the egg.
• Gametes contain a mix of genetic material to create offspring with unique genetic profiles (genetic variation).

Variation

• Species variation results from genetic (inheritance) or environmental factors.
  • Genetic variation: eye color, hair color, genetically inherited disorders, blood groups.
  • Environmental variation: sunlight exposure, moisture levels, temperature, soil mineral content.

Natural Selection

• Evolution: Evolution is change in the range of organisms on Earth over time.
• Natural selection: Process where new species emerge from existing ones.
• Charles Darwin's theory: species vary in genes, better-suited individuals are more likely to survive, reproduce, and pass on genes.
• Natural selection: mutations cause variation, best adapted ones thrive, reproduce and pass on the gene, increasing population.